1. Technical Field
This invention relates to a wide field of view optical system and, more particularly, to an all-reflective afocal three-mirror anastigmat optical system.
2. Discussion
Wide field of view optical systems, such as reflective, afocal telescopes are utilized in multispectral applications and in certain space sensor systems. The use of these wide field of view optical systems is to provide large unobscured two-dimensional fields of view. The image space field of view may be viewed at the exit pupil by various types of scanning or staring sensors which consist generally of an imaging optical system and a detector array. This information may be separated by dichroic beamsplitters and used by several sensors or imaging optics with detector arrays operating in different spectral bands of interest.
An example of a three-mirror all-reflective afocal telescope is illustrated by U.S. Pat. No. 3,674,334 issued Jan. 4, 1971 to Offner entitled, "Catoptric Anastigmatic Afocal Optical System". The patent illustrates a series of variations on a three-mirror reimaging telescope with a mirror power distribution that is positive-negative-positive. While these designs appear to be useful for certain applications, they are not capable of wide fields of view nor are they well suited for low magnifications.
An all-reflective afocal telescope like that illustrated in U.S. Pat. No. 4,804,258 issued Feb. 14, 1989 to Kebo, entitled, "Four-Mirror Afocal Wide Field of View Optical System", assigned to the same assignee as the present invention, the specification of which is herein expressly incorporated by reference, illustrates a four-mirror reimaging telescope with a mirror power distribution that is positive-negative-positive-positive. While this design has more field of view capability than Offner, there is additional complexity of the fourth mirror and this design is not well suited for low magnifications.
In scanning FLIR systems where a wide field of view is required more so than high resolution (eg., for application involving pilotage, navigation, driving), low magnification afocal telescopes are typically employed in front of the scanner, imager, and detector/dewar. These afocal telescopes often have magnifications near or less than unity in order to increase the object space field of view over that generated by the scanner and imager. Object space fields of 20.times.30, 20.times.40, and 30.times.40 degrees are typical for these applications. In the past, these applications have been exclusively performed by refractive systems.
Various types of refractive systems provide large field of view capabilities at low magnifications, however, these refractive systems have several disadvantages. Some limitations of refractive telescopes are chromatic aberrations, spectral limitations (eg., visible TV system cannot share the same telescope as the LWIR FLIR), defocus with temperature change requiring compensation, potentially high narcissus, and high cost associated with the complexity and expensive refractive materials.